If a Vicat apparatus is not at hand, sub stantially the same result may be obtained as follows: Mix the paste to such a consistency that if a ball of mortar about 2 inches in diameter be dropped upon a stone slab or glass plate from a height of 20 inches, it will not crack nor flatten to less than half of its original diameter. This is a simple, rapid, and reason ably accurate method of identifying a certain degree of plasticity, and gives a consistency formerly much employed; but to secure the normal consistency required by the standard American speci fication, first determine the plasticity by the ball method, and then in making pastes for the standard tests use the amount of water required by the ball method plus 1 per cent of the weight of the dry cement.
Neither of the above methods of determining plasticity is applicable to mixtures of sand and cement,—the first because the sand grains interfere with the penetration of the rod, and the second because the mortar is so deficient in cohesion that the ball will not hold its shape when dropped on the stone slab. The only method of determining the normal consistency of mortar is to compare it by the eye and under the trowel with neat cement paste of normal consistency determined as above. This has been done by a number of experts under the direction of the Committee on Uniform Tests of Cement of the American Society of Civil Engineers with the results shown in Table 11, when the percentage for neat cement has been determined as described in § 162.
Table 11, which may properly be called the American standard, applies only to a 1 : 3* mortar; but it sometimes happens in experimental work that other proportions of cement and sand are used, and then it is desirable to know the per cent of water required to produce normal consistency. In this case Table 12 may be used. This table is based upon a formula prepared by Ferett, and was recommended for temporary use by the Committee of the American Society for Testing Materials awaiting the results of further investi gations by that society. Notice that Table 11 gives slightly smaller results for a 1 : 3 mortar than Table 12, page 76.
The sand and the cement should be thoroughly mixed dry, and the water required to reduce the mass to the proper consistency should be added all at once. The mixing should be prompt and thorough. The mass should not be simply turned, but the mortar should be rubbed against the top of the slate or glass mixing-table with the ball of the hand or a trowel. Insufficient working greatly decreases the strength of the mortar— frequently one half. With a slow-setting cement a kilogram of the dry materials should be strongly and rapidly rubbed for not less than 5 minutes, when the consistency should be such that it will not be changed by an additional mixing for 3 minutes.
A variety of machines for mixing mortar for experimental pur poses have been devised,* but none has proved even fairly successful.
In 1885 a Committee of the American Society of Civil Engineers recommended a form of briquette which has been the standard in this country until recently. The present standard, Fig. 2, is the same as the former one except that the corners of the briquette are rounded off to facilitate its removal from the moulds. Practically the same form is used in England; and the form employed in continental Europe is somewhat similar to the above, except that the section is 5 square centimeters (0.8 square inch) and the reduction to produce the minimum section is by very much more abrupt curves. The continental form gives only 70 to 80 per cent as much strength as the American form.
The moulds should be of brass or some non corrodible material, and should have sufficient metal on the sides to prevent spreading during the filling of the moulds. They may be single or multiple, the latter being preferable where a great number of bri quettes are required, since the greater quantity of mortar that can be mixed at once tends to produce greater uni formity in the results. The moulds are in two parts, to facilitate removal of the briquette without breaking it. The moulds should be cleaned and wiped with an oily rag before being used.
mediately after having worked the paste or mortar to the proper consistency, it should be placed in the briquette moulds by hand. "The moulds should be filled at once, the material being pressed in firmly with the fingers and smoothed off with a trowel without ram ming. The material should be heaped up on the upper surface of the mould; and, in smoothing off, the trowel should be drawn over the mould in such a manner as to exert a moderate pressure on the excess material. The mould should then be turned over and the operation repeated." * "A check upon the uniformity of the mixing and moulding is afforded by weighing the briquettes just prior to immersion, or upon removal from the moist closet. Briquettes which vary in weight more than 3 per cent from the average should not be tested."* Several machines have been made for moulding the bri quettes; f but all are very slow, and none permit of moulding more than one briquette at a time, and none are practicable with pastes or mortars of the consistency recommended for American practice.